C++: get value of memory address from a text file

I have a text file with a single line that stores the memory address to a variable (of type double) being used in the code.

Content of temp.txt: 0x7f3c00844c00

I would like to read this line and dereference it to get the double value stored at the address (i.e. 0x7f3c00844c00)

This is what I tried:

std::fstream temp;
temp.open("temp.txt");
string address;
getline(temp, address);
temp.close();
std::cout << *strtoul(address, NULL, 16) << std::endl;

That does not work --> "cannot convert std::string to const char *"

Any help much appreciated!

Your call to strtoul() is wrong for 3 reasons:

• it takes a C-style null-terminated const char* string as input, not a std::string object. Hence the compiler error you are seeing. Use the std::string::c_str() method, or the std::stoul() function, to solve this.

• it returns an integer, not a pointer. Use)reinterpret_cast` to solve this.

• the integer value it returns may or may not fit in a pointer, depending on which platform this code is running on. So, you are risking truncating the resulting pointer. Use std::strtoull() when running on a 64-bit system.

Try this instead:

std::ifstream ifs("temp.txt");
std::string str;
std::getline(ifs, str);
temp.close();
uintptr_t address = std::strtoul(str.c_str(), NULL, 16); // or std::strtoull()
double *pdbl = reinterpret_cast<double*>(address);
std::cout << *pdbl << std::endl;

That being said, consider using operator>> instead, as it has an overload for reading in a pointer, eg:

std::ifstream ifs("temp.txt");
void* address;
ifs >> address;
ifs.close();
double *pdbl = static_cast<double*>(address);
std::cout << *pdbl << std::endl;